<style>
  * {
    margin: 0;
    padding: 0;
  }
  ul {
    display: flex;
    border-bottom: 2px solid #e01222;
    padding: 0 10px;
  }
  li {
    width: 100px;
    height: 50px;
    line-height: 50px;
    list-style: none;
    text-align: center;
  }
  li a {
    display: block;
    text-decoration: none;
    font-weight: bold;
    color: #333333;
  }
  li a.active {
    background-color: #e01222;
    color: #fff;
  }

</style>

<div id="app">
  <ul>
    <!-- 小技巧  利用等式判断返回布尔值 -->
    <!-- 点击的时候改变下标 -->
    <li v-for="(item,index) in list" :key="item.id" @click="activeIndex = index">
      <a :class="{active: index === activeIndex}" href="#">{{item.name}}</a>
    </li>
  </ul>
</div>
<script src="https://cdn.jsdelivr.net/npm/vue@2/dist/vue.js"></script>
<script>
  const app = new Vue({
    el: '#app',
    data: {
      list: [
        { id: 1, name: '京东秒杀' },
        { id: 2, name: '每日特价' },
        { id: 3, name: '品类秒杀' }
      ],
      activeIndex: 0
    }
  })
</script>